125 lines
4.9 KiB
Markdown
125 lines
4.9 KiB
Markdown
# Altair -- Fork Logic
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## Table of contents
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<!-- START doctoc generated TOC please keep comment here to allow auto update -->
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<!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE -->
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- [Introduction](#introduction)
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- [Configuration](#configuration)
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- [Helper functions](#helper-functions)
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- [Misc](#misc)
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- [`compute_fork_version`](#compute_fork_version)
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- [Fork to Altair](#fork-to-altair)
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- [Fork trigger](#fork-trigger)
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- [Upgrading the state](#upgrading-the-state)
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<!-- END doctoc generated TOC please keep comment here to allow auto update -->
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## Introduction
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This document describes the process of the first upgrade of the beacon chain: the Altair hard fork, introducing light client support and other improvements.
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## Configuration
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| Name | Value |
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| - | - |
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| `ALTAIR_FORK_VERSION` | `Version('0x01000000')` |
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| `ALTAIR_FORK_EPOCH` | `Epoch(74240)` (Oct 27, 2021, 10:56:23am UTC) |
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## Helper functions
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### Misc
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#### `compute_fork_version`
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```python
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def compute_fork_version(epoch: Epoch) -> Version:
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"""
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Return the fork version at the given ``epoch``.
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"""
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if epoch >= ALTAIR_FORK_EPOCH:
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return ALTAIR_FORK_VERSION
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return GENESIS_FORK_VERSION
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```
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## Fork to Altair
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### Fork trigger
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The fork is triggered at epoch `ALTAIR_FORK_EPOCH`.
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Note that for the pure Altair networks, we don't apply `upgrade_to_altair` since it starts with Altair version logic.
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### Upgrading the state
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If `state.slot % SLOTS_PER_EPOCH == 0` and `compute_epoch_at_slot(state.slot) == ALTAIR_FORK_EPOCH`, an irregular state change is made to upgrade to Altair.
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The upgrade occurs after the completion of the inner loop of `process_slots` that sets `state.slot` equal to `ALTAIR_FORK_EPOCH * SLOTS_PER_EPOCH`.
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Care must be taken when transitioning through the fork boundary as implementations will need a modified [state transition function](../phase0/beacon-chain.md#beacon-chain-state-transition-function) that deviates from the Phase 0 document.
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In particular, the outer `state_transition` function defined in the Phase 0 document will not expose the precise fork slot to execute the upgrade in the presence of skipped slots at the fork boundary. Instead the logic must be within `process_slots`.
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```python
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def translate_participation(state: BeaconState, pending_attestations: Sequence[phase0.PendingAttestation]) -> None:
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for attestation in pending_attestations:
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data = attestation.data
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inclusion_delay = attestation.inclusion_delay
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# Translate attestation inclusion info to flag indices
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participation_flag_indices = get_attestation_participation_flag_indices(state, data, inclusion_delay)
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# Apply flags to all attesting validators
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epoch_participation = state.previous_epoch_participation
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for index in get_attesting_indices(state, attestation):
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for flag_index in participation_flag_indices:
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epoch_participation[index] = add_flag(epoch_participation[index], flag_index)
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def upgrade_to_altair(pre: phase0.BeaconState) -> BeaconState:
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epoch = phase0.get_current_epoch(pre)
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post = BeaconState(
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# Versioning
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genesis_time=pre.genesis_time,
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genesis_validators_root=pre.genesis_validators_root,
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slot=pre.slot,
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fork=Fork(
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previous_version=pre.fork.current_version,
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current_version=ALTAIR_FORK_VERSION,
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epoch=epoch,
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),
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# History
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latest_block_header=pre.latest_block_header,
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block_roots=pre.block_roots,
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state_roots=pre.state_roots,
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historical_roots=pre.historical_roots,
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# Eth1
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eth1_data=pre.eth1_data,
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eth1_data_votes=pre.eth1_data_votes,
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eth1_deposit_index=pre.eth1_deposit_index,
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# Registry
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validators=pre.validators,
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balances=pre.balances,
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# Randomness
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randao_mixes=pre.randao_mixes,
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# Slashings
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slashings=pre.slashings,
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# Participation
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previous_epoch_participation=[ParticipationFlags(0b0000_0000) for _ in range(len(pre.validators))],
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current_epoch_participation=[ParticipationFlags(0b0000_0000) for _ in range(len(pre.validators))],
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# Finality
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justification_bits=pre.justification_bits,
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previous_justified_checkpoint=pre.previous_justified_checkpoint,
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current_justified_checkpoint=pre.current_justified_checkpoint,
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finalized_checkpoint=pre.finalized_checkpoint,
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# Inactivity
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inactivity_scores=[uint64(0) for _ in range(len(pre.validators))],
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)
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# Fill in previous epoch participation from the pre state's pending attestations
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translate_participation(post, pre.previous_epoch_attestations)
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# Fill in sync committees
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# Note: A duplicate committee is assigned for the current and next committee at the fork boundary
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post.current_sync_committee = get_next_sync_committee(post)
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post.next_sync_committee = get_next_sync_committee(post)
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return post
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```
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